In recent years, there has been a general reconsideration of using fossil fuels as a primary source of energy for transportation vehicles, due to environmental, economic and geopolitical issues. This reconsideration has squarely focused on the development of electric vehicle and hybrid electric vehicle platforms as possible solutions. These vehicles have only been given a serious look as a replacement to fossil fuel vehicles lately, mainly because of advancements in motor and electronics technology and battery technology.
Electrical motors can take two types of forms: DC motors or AC motors. DC motors have been developed and used extensively for a long period of time due to their high performance in motion and drive applications. However, with DC motors there are many maintenance and efficiency issues due to inclusion of slip rings and brushes that are needed to commutate these machines. With the more recent development of power electronics, new control technologies and machine topologies, great progress has been made to replace these DC machines in the variable speed drive area. AC motors are used to obtain better performance, reliability, improved maintenance characteristics, and overall lower costs. Extensive research and development has gone into developing AC machines that are suitable for drive applications and still match the drive characteristics of their DC counterparts.
AC motors are designed for use with either polyphase or single-phase power systems. AC motors are typically divided into these categories: series, synchronous, and induction motors. Induction motors, single-phase or polyphase, are the most commonly used type of AC motor and the name is derived from the fact that AC voltages are induced in a rotor circuit by rotating in a magnetic field of a stator. Currently, induction machines are the dominant choice for both constant speed and variable speed drives. However, induction machines also have difficulties. For instance, since rotor windings are present in all induction machines, the rotor current produces rotor resistive losses, decreasing the efficiency of the motor, particularly at low power ratings, and, in some cases, causing cooling problems.
In light of the drawbacks that are inherent in induction machines, more attention has been given to the permanent magnet machines which greatly increase power density and torque density. Even more power density is possible in such PM motor topologies as hybrid axial-radial motors (HARMs) having permanent magnets which providing fields in multiple planes. In these machines, the rotor field flux is established by permanent magnets. It is known in this field that high power and torque density as well as high efficiency are some of the most desirable characteristics for electrical machines. Improvements to these characteristics have been one of the main aspects of research on electrical machines in the last couple of decades.
Various implementations of hybrid axial-radial motors (HARMs) have been documented in U.S. Pat. No. 7,034,422 and in a paper by A. G. Jack, “Permanent-Magnet Machines with Powdered Iron Cores and Prepressed Windings,” IEEE Trans. Industry Applications, vol. 36, no. 4, pp 1077-1084, Jul./Aug. 2000. In the paper, the stator winding is achieved through a toroidal type of winding, or a winding that is wrapped around a torrus-shaped stator. The '422 patent describes several HARM embodiments related to multiple stators and/or multiple rotors with the same or different machine types representing either the radial or axial portion of the HARMs. It also refers to the use of multiple machine types packaged into one radial-axial flux machine as a means of fault tolerance or reliability. The '422 patent describes a specific winding and stator geometry to enhance the overall performance of radial/Axial flux machines. However, there is still a need for highly efficient and power dense motors to make them a viable solution to various applications while the production and manufacturing cost remains low.